The present invention relates to a magnetron useful in, for example, a microwave oven or the like, to generate microwaves.
For example, as described in Japanese Laid Open Patent Publication 6-97595 (H01J23/04, H01J23/14), a conventional microwave oven magnetron has a cathode portion having the two ends of a coiled filament fastened onto a pair of end hats. Cathode leads formed from molybdenum (Mo) and the like are affixed to these end hats. These cathode leads extend to the exterior by passing via through holes of a ceramic stem. A tubular metal container is soldered with silver soldering, or the like, onto a metallized surface on the outer perimeter of this ceramic stem. A separately assembled anode part, which is not shown, is affixed to the tubular metal container. The cathode leads are sealed in an airtight manner to the metallized surface of the ceramic stem by a silver soldering material with a metal joining plate as the medium material.
With a magnetron of this construction, in general, the tubular metal container that is joined with the anode part has a ground potential. On the other hand, a negative high voltage of 4 kV, for example, is applied and operated on the cathode part, constructed from the filament and the cathode leads and the like. Therefore, a discharge can easily occur between the end of the tubular metal container, which is soldered onto the metallized surface on the perimeter of the ceramic stem, and the metal joining plate, which is used when soldering the cathode leads onto the ceramic stem. This is because these joining parts both have metallized surfaces, and the edges become rough. In addition, the silver soldering used in the air tight seal grows at the edges, and numerous needle-like protrusions are formed. These become needle-like electrodes and also narrow the spacing.
Particularly with a microwave oven using a leakage transformer, when the power is turned on without pre-heating the filament, in the initial stage where electrons are not being emitted from the filament, a no-load voltage of 8-10 kV is added to the magnetron. Discharge occurs at the metallized part having the airtight seal as described above. A surge voltage is induced, and the high voltage parts are destroyed.
In order to solve these problems, a ring-shaped depressed groove is formed between the metallized surface that joins to the tubular metal container and the metallized surface that joins to the cathode leads. By having the tubular metal container and the metal joining plate protrude over the upper surface of the ring-shaped depressed groove, the needle-like protrusions of the soldering material are electrically covered with an electric conductor, and a field-less layer is formed to prevent discharges.
However, with a construction with the above approach, because the tubular metal container and the metal joining plate protrude over the upper surface of the ring-shaped depressed groove, the shapes of the tubular metal container and the metal joining plate become larger, and the costs increase. In addition, because the space between the tubular metal container and the metal joining plate is narrowed, discharges is not completely prevented.
It is an object of the present invention to provide a magnetron which overcomes the foregoing problems.
It is a further object of the present invention to provide a magnetron that reliably prevents discharges generated between the joining surface of the stem insulating material with the tubular metal container and the joining surface with the cathode leads.
According to an embodiment of the present invention a magnetron comprises a tubular metal container, joined in an airtight manner with an anode part, constructing one section of a vacuum container. A stem insulating material has a perimeter which is joined in an airtight manner to an open end of the tubular metal container. A cathode has a filament positioned at a central axis of the anode part. A pair of cathode leads support the cathode and are affixed to a metal joining plate that is joined in an airtight manner to a central part of the stem insulating material. A ring-shaped depressed groove is formed between a joining surface of the stem insulating material with the tubular metal container and a joining surface with the cathode leads. A metallized layer, formed at the joining surface of the tubular metal container and the joining surface of the cathode lead, is positioned separated from the edge of the ring-shaped depressed groove. At least one or the other of the open end of the tubular metal container or the metal joining plate protrude towards the interior more than the metallized layer.
A magnetron having the above construction results in the metallized layer not being formed on the joining surface at the edge of the ring-shaped depressed groove. As a result, even if a silver soldering material, or the like, used in the air-tight seal, form needle-like protrusions at the edges of the metallized layer, the needle-like protrusions are not formed at the edge of the ring-shaped depressed groove. Therefore, without narrowing the space between the open end of the tubular metal container and the metal joining plate, the needle-like protrusions formed at the edges of the metallized layer can be electrically covered with a metal conductor to form a field-free layer. As a result, in the initial stage, before electrons are emitted from the cathode, even if a no-load voltage of 8-10 kV is applied on the cathode, discharge is reliably prevented.
In addition, the joining surface of the tubular metal container and the joining surface of the cathode leads are positioned preferably on the same plane. The metallized layer is formed preferably by pattern printing. As a result, because the edges of the ring-shaped depressed groove are on the same plane as the joining surfaces and can be easily excluded from the coating area, the metallized layer is formed without any decline in the coating operation.
According to another embodiment of the present invention, a magnetron, comprises a tubular metal container joined in an airtight manner with an anode part, constructing one section of a vacuum container. A stem insulating material has a perimeter which is joined in an airtight manner to an open end of the tubular metal container. A cathode has a filament positioned at a central axis of the anode part. A pair of cathode leads support the cathode and are affixed to a metal joining plate that is joined in an airtight manner to a central part of the stem insulating material. A ring-shaped depressed groove is formed between a joining surface of the stem insulating material with the tubular metal container and a joining surface with the cathode leads. A metallized layer is formed at the joining surfaces. A step part is lower than the joining surfaces being formed at the edge of the ring-shaped depressed groove.
A magnetron having the above construction has a step part which can stop the growth of the silver soldering material used for the air-tight seal. As a result, the needle-like protrusions are no longer formed at the edge of the ring-shaped depressed groove. Without narrowing the space between the open end of the tubular metal container and the metal joining plate, the needle-like protrusions can be electrically covered by a metal conductor ahead of the depressed groove, and a field-free layer is formed. As a result, in the initial stage before electrons are emitted from the cathode, even if a no-load voltage of 8-10 kV is applied on the cathode, discharge is reliably prevented.
In addition, the joining surface of the tubular metal container and the joining surface of the cathode leads are positioned preferably along the same plane. As a result, forming of the metallized layer, which is necessary for soldering to the joining surface and is formed by coating molybdenum (Mo) and manganese (Mn), can be conducted by a single screen coating. The production quality of the stem insulating material is improved.
The above, and other objects, features, and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.